Fix compatibility with python3 in putFile method

in python3 os.read returns byte object not a string which caused
writing loop to be endless

by adding b'' comparison we hold compatibility both with python2
and python3

Author: TomasKorbar

cupspk.py

@@ -448,7 +448,7 @@ def putFile(self, *args, **kwds):
             if fd is not None:
                 os.lseek (fd, 0, os.SEEK_SET)
                 buf = os.read (fd, 512)
-                while buf != '':
+                while buf != '' and buf != b'':
                     os.write (tmpfd, buf)
                     buf = os.read (fd, 512)
             else: